Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. An analogous class of fluids are the electrorheological (ER) fluids which exhibit a like ability to change their flow or rheological characteristics under the influence of an applied electric field. In both instances, these induced rheological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological or electrorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable powertrain and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about one Tesla. At the present state of development, MR fluids appear to offer significant advantages over ER fluids, particularly for automotive applications, because the MR fluids are less sensitive to common contaminants found in such environments, and they display greater differences in rheological properties in the presence of a modest applied field.
Since MR fluids contain noncolloidal solid particles which are often seven to eight times more dense than the liquid phase in which they are suspended, suitable dispersions of the particles in the fluid phase must be prepared so that the particles do not settle appreciably upon standing nor do they irreversibly coagulate to form aggregates. Examples of suitable magnetorheological fluids are illustrated, for example, in U.S. Pat. Nos. 4,957,644 issued Sep. 18, 1990, entitled "Magnetically Controllable Couplings Containing Ferrofluids"; 4,992,190 issued Feb. 12, 1991, entitled "Fluid Responsive to a Magnetic Field"; 5,167,850 issued Dec. 1, 1992, entitled "Fluid Responsive to a Magnetic Field"; 5,354,488 issued Oct. 11, 1994, entitled "Fluid Responsive to a Magnetic Field"; and 5,382,373 issued Jan. 17, 1995, entitled "Magnetotheological Particles Based on Alloy Particles".
As suggested in the above patents and elsewhere, a typical MR fluid in the absence of a magnetic field has a readily measurable viscosity that is a function of its vehicle and particle composition, particle size, the particle loading, temperature and the like. However, in the presence of an applied magnetic field, the suspended particles appear to align or cluster and the fluid drastically thickens or gels. Its effective viscosity then is very high and a larger force, termed a yield stress, is required to promote flow in the fluid.
Given a stable noncoagulating suspension, the problem in formulating useful MR fluids as working media in actuators such as shock absorbers, powertrain mounts, starting clutches and the like can be stated as follows. The off-state viscosity of the fluid (that is, the viscosity with no magnetic field applied) is to be minimized or, alternatively, fixed at a constant acceptable value while the on-state (magnetic field applied) yield stress of the fluid is to be maximized or fixed at an acceptably constant value. Thus, the off-state viscosity and the on-state yield stress are both important because they both contribute to the magnitude of a magnetorheological effect. The difference between such off-state viscosity and on-state yield stress may be conveniently expressed as a "turn-up ratio". Turn-up ratio is defined as the ratio of the force or torque output generated by the magnetically activated MR fluid divided by the force or torque output for the same fluid in the unactivated or off-state. In MR fluids, the maximum force or torque "on" is controlled by the yield stress while the minimum force or torque "off" is controlled by the viscosity. The object in designing controllable fluid actuators is generally to maximize the turn-up ratio under given operating conditions. It is an object of the present invention to manipulate the material or fluid composition variables so as to maximize the turn-up ratio of the fluid.